cgo: add GoBytes, fix gmp example

Fixes #1640.
Fixes #2007.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4815063

misc/cgo/gmp/gmp.go

@@ -265,7 +265,7 @@ func (z *Int) Mod(x, y *Int) *Int {
 func (z *Int) Lsh(x *Int, s uint) *Int {
 	x.doinit()
 	z.doinit()
-	C.mpz_mul_2exp(&z.i[0], &x.i[0], C.ulong(s))
+	C.mpz_mul_2exp(&z.i[0], &x.i[0], C.mp_bitcnt_t(s))
 	return z
 }
 
@@ -273,7 +273,7 @@ func (z *Int) Lsh(x *Int, s uint) *Int {
 func (z *Int) Rsh(x *Int, s uint) *Int {
 	x.doinit()
 	z.doinit()
-	C.mpz_div_2exp(&z.i[0], &x.i[0], C.ulong(s))
+	C.mpz_div_2exp(&z.i[0], &x.i[0], C.mp_bitcnt_t(s))
 	return z
 }
 

misc/cgo/stdio/file.go

@@ -42,3 +42,4 @@ func (f *File) Flush() {
 }
 
 var Greeting = C.GoString(C.greeting)
+var Gbytes = C.GoBytes(unsafe.Pointer(C.greeting), C.int(len(Greeting)))

src/cmd/cgo/ast.go

@@ -189,6 +189,10 @@ func (f *File) saveExport(x interface{}, context string) {
 			error(c.Pos(), "export missing name")
 		}
 
+		if name != n.Name.Name {
+			error(c.Pos(), "export comment has wrong name %q, want %q", name, n.Name.Name)
+		}
+
 		f.ExpFunc = append(f.ExpFunc, &ExpFunc{
 			Func:    n,
 			ExpName: name,

src/cmd/cgo/doc.go

@@ -52,6 +52,7 @@ C.char, C.schar (signed char), C.uchar (unsigned char),
 C.short, C.ushort (unsigned short), C.int, C.uint (unsigned int),
 C.long, C.ulong (unsigned long), C.longlong (long long),
 C.ulonglong (unsigned long long), C.float, C.double.
+The C type void* is represented by Go's unsafe.Pointer.
 
 To access a struct, union, or enum type directly, prefix it with
 struct_, union_, or enum_, as in C.struct_stat.
@@ -68,6 +69,21 @@ C compilers are aware of this calling convention and adjust
 the call accordingly, but Go cannot.  In Go, you must pass
 the pointer to the first element explicitly: C.f(&x[0]).
 
+A few special functions convert between Go and C types
+by making copies of the data.  In pseudo-Go definitions:
+
+	// Go string to C string
+	func C.CString(string) *C.char
+
+	// C string to Go string
+	func C.GoString(*C.char) string
+
+	// C string, length to Go string
+	func C.GoStringN(*C.char, C.int) string
+
+	// C pointer, length to Go []byte
+	func C.GoBytes(unsafe.Pointer, C.int) []byte
+
 Cgo transforms the input file into four output files: two Go source
 files, a C file for 6c (or 8c or 5c), and a C file for gcc.
 

src/cmd/cgo/gcc.go

@@ -1140,6 +1140,14 @@ func (c *typeConv) Type(dtype dwarf.Type) *Type {
 			t.Align = c.ptrSize
 			break
 		}
+		if dt.Name == "_GoBytes_" {
+			// Special C name for Go []byte type.
+			// Knows slice layout used by compilers: pointer, length, cap.
+			t.Go = c.Ident("[]byte")
+			t.Size = c.ptrSize + 4 + 4
+			t.Align = c.ptrSize
+			break
+		}
 		name := c.Ident("_Ctypedef_" + dt.Name)
 		t.Go = name // publish before recursive call
 		sub := c.Type(dt.Type)

src/cmd/cgo/out.go

@@ -236,7 +236,7 @@ func (p *Package) writeDefsFunc(fc, fgo2 *os.File, n *Name) {
 	printer.Fprint(fgo2, fset, d)
 	fmt.Fprintf(fgo2, "\n")
 
-	if name == "CString" || name == "GoString" || name == "GoStringN" {
+	if name == "CString" || name == "GoString" || name == "GoStringN" || name == "GoBytes" {
 		// The builtins are already defined in the C prolog.
 		return
 	}
@@ -316,7 +316,7 @@ func (p *Package) writeOutput(f *File, srcfile string) {
 
 func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 	name := n.Mangle
-	if name == "_Cfunc_CString" || name == "_Cfunc_GoString" || name == "_Cfunc_GoStringN" || p.Written[name] {
+	if name == "_Cfunc_CString" || name == "_Cfunc_GoString" || name == "_Cfunc_GoStringN" || name == "_Cfunc_GoBytes" || p.Written[name] {
 		// The builtins are already defined in the C prolog, and we don't
 		// want to duplicate function definitions we've already done.
 		return
@@ -646,6 +646,8 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 			}
 			return r
 		}
+		error(e.Pos(), "unrecognized Go type %s", t.Name)
+		return &Type{Size: 4, Align: 4, C: c("int")}
 	case *ast.SelectorExpr:
 		id, ok := t.X.(*ast.Ident)
 		if ok && id.Name == "unsafe" && t.Sel.Name == "Pointer" {
@@ -679,8 +681,10 @@ __cgo_size_assert(double, 8)
 
 const builtinProlog = `
 typedef struct { char *p; int n; } _GoString_;
+typedef struct { char *p; int n; int c; } _GoBytes_;
 _GoString_ GoString(char *p);
 _GoString_ GoStringN(char *p, int l);
+_GoBytes_ GoBytes(void *p, int n);
 char *CString(_GoString_);
 `
 
@@ -704,6 +708,13 @@ void
 	FLUSH(&s);
 }
 
+void
+·_Cfunc_GoBytes(int8 *p, int32 l, Slice s)
+{
+	s = runtime·gobytes((byte*)p, l);
+	FLUSH(&s);
+}
+
 void
 ·_Cfunc_CString(String s, int8 *p)
 {

src/pkg/runtime/runtime.h

@@ -413,6 +413,7 @@ void*	runtime·mal(uintptr);
 String	runtime·catstring(String, String);
 String	runtime·gostring(byte*);
 String  runtime·gostringn(byte*, int32);
+Slice	runtime·gobytes(byte*, int32);
 String	runtime·gostringnocopy(byte*);
 String	runtime·gostringw(uint16*);
 void	runtime·initsig(int32);

src/pkg/runtime/string.goc

@@ -74,6 +74,16 @@ runtime·gostringn(byte *str, int32 l)
 	return s;
 }
 
+Slice
+runtime·gobytes(byte *p, int32 n)
+{
+	Slice sl;
+
+	sl.array = runtime·mallocgc(n, FlagNoPointers, 1, 0);
+	runtime·memmove(sl.array, p, n);
+	return sl;
+}
+
 String
 runtime·gostringnocopy(byte *str)
 {